I think the first step you should take is test the water for everything including metals so you will know what is going on. Also my understanding is that ascorbic acid is very pH dependant for it's use as a stain remover. I am not that familiar with it's use. I know several companies market it as a stain remover (reading a MSDS will usually tell you what is in the product). Perhaps the second step is to do a bit of internet searching and looking at the various products and what is in them and how they are used. Above all, DON"T GIVE UP! Like I said before I am not an expert on stains in pools but I know that there are several on this board! I'm just a guy with a little knowledge and a big mouth!